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Attosecond Observations Probe Chemical Reactions

Photonics SpectraDec 2015
ZURICH — Laser pulses have been used to track the movements of electrons in molecules with a time resolution of 100 attoseconds — and even to alter their behavior.

Demonstrated by researchers at the Swiss Federal Institute of Technology (ETH Zurich) and described by theoreticians from Belgium, Canada, Denmark and Russia, the results point to a potential method for controlling chemical reactions.

This image shows electron density in a molecule. Courtesy of the Moscow Institute of Physics and Technology.
"The study observed the migration of electrons along a linear molecule," said Oleg Tolstikhin of the Moscow Institute of Physics and Technology. "We were able to see, for the first time, the movement of electrons, how it all happens, in detail. In addition, we demonstrated that this movement can be controlled, and therefore it is theoretically possible to control the outcome of chemical reactions."

Formation of chemical bonds is governed by the redistribution of electrons within and across molecules. Therefore tracking the ultrafast movement of electrons — or more precisely the restructuring of molecules' electron shells — is key to understanding chemical and biochemical reactions.

In their experiment the researchers used molecules of iodoacetylene (HCCI), which are elongated chains of four atoms — hydrogen, two carbons and an iodine.

Under the effect of powerful and very short laser pulses, the molecule's electron shell configuration changed: A hole appeared and began to oscillate, moving from one end of the molecule to the other.

Tolstikhin emphasized that it is not movement in a literal sense, as in classical physics.

"As a result of tunneling ionization in a strong laser field, a superposition of two quantum states of the hole occurs," he said. "It is similar to Schrödinger's cat that is simultaneously alive and dead. In this superposition. the hole can be found at opposite ends of the molecule at the same time. The probability of finding a hole at either end oscillates over time, which is what creates the effect of the hole migrating along the molecule. The hole moves from one end to the other and the characteristic time taken for this movement is approximately 100 attoseconds."

By irradiating oriented molecules with powerful laser pulses, the researchers obtained high-harmonic spectra reflecting the state of a molecule's electron shell. The theoreticians' task was to isolate information about this dynamic from the data obtained, and learn how to decipher the spectra.

"In reality we are observing not the position of the electrons, but rather the high-harmonic spectrum which occurs during the process of interaction between a powerful laser pulse and a molecule," Tolstikhin said. "Using these spectra, which are indirectly linked to the movement of the hole, its position can be restored, which is what we have done here."

In addition, by altering the laser polarization, the researchers demonstrated the ability to influence the dynamics of the restructuring of a molecule's electron shell.

"This is what will ultimately allow the outcome of chemical reactions to be controlled," Tolstikhin said. "If you have a mixture where chemical reactions can result in different outcomes, you can, by choosing the required pulse shapes, select the desired outcome."

Funding came from the Ministry of Education and Science of the Russian Federation.